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Listening to KSEA, what does orange, green, or blue line mean?
I'm listening to an LiveATC stream for the Seattle Tacoma airport.
I keep hearing things like "push approved on the blue line".
I'm pretty sure I've heard orange, blue, and green as colors.
What does this mean?

If Seattle works anything like a number of major German airports, these instructions refer to literal coloured lines on the ground.
Taxiway markings are usually yellow, but some large airports have started to use different colours for marking taxilanes on the apron ("ramp"). This is a way of maximizing use of available space in areas used by many differently sized aircraft.
One example is Munich, Germany (EDDM) where yellow, orange and blue taxi lines are used.

(snippet from APDC EDDM, from DFS)
Two medium sized aircraft can pass eachother when following the blue and orange lines, whereas a larger aircraft will have to follow the central yellow line, thus taking up the entire width of the taxiway.
As @ymb1 pointed out in a comment, the system in KSEA appears to be similar. The colour of the taxilines are varied, and signs (or markings on the ground) describe the colour code for certain taxi lines:

It looks like all of the actual taxilanes on the ramp at SEA are the same color. There are some different colors for the lines directing traffic into/out of some of the stands, though, so you're probably right. However, I don't see any blue lines; just orange, red, or dashed green/orange. And the red ones only appear to be at a few remote stands, not at the passenger gates. Of course, it's possible that Google's imagery is out-of-date.

@ymb1 Ah, nice find. I guess the imagery I was seeing on Google Maps is indeed old then, as I don't see those lines there.


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Are there fixed wing planes that have a Vne above their terminal velocity with the nose pointed straight down?
Are there any fixed wing aircraft that will not exceed Vne if they are taken to their ceiling, engine is pulled to idle or shut down, and then the nose is pointed straight down? Lets assume they are going to pull out of this dive to level flight above sea level without overloading the airframe, which clearly limits the time/distance of the vertical dive.
Are there any aircraft that would not exceed their Vne (and engine speed limits!) in the same scenario as above, but with the engine controls at full power?
If there are aircraft where this is true, are they military fighter aircraft, or are there other (surprising) examples of civilian aircraft where this is true?
This question comes from the realization that aerobatics (and spins, and things like spatial disorientation) can lead to an attitudes with the nose pitched quite down, and in that attitude speed builds quite quickly in many aircraft to speeds above Va, which means pulling out of the dive could easily overstess the airframe. At the same time, not pulling out could also cross Vne and probably continue to increase through any design margin to where the airframe is also overstressed by "forward" (downward) airspeed. Which got me to wondering "are there 1-2 person aerobatic training aircraft out there where this is just not a concern, because the airframe is strong enough and has so much drag?"

OK. Added an edit. Though I'm OK with the answer being "yes, but only supersonic military aircraft such as the F##".

The only aircraft I'm aware of that are Vne limited are gliders with "speed limiting" dive brakes. You can't exceed Vne with them fully extended in any attitude.

@JohnK I've heard the same about gliders, although I've never been brave enough to test. Always seems crazy to me that those little slats out there are so draggy (and strong) that the entire airplane can hang off of them.

Did we ever get an answer here, leaving out the "full power" part, other than gliders with speed brakes deployed, and dive bombers with speed brakes deployed?  How about a Stearman at idle power?  How a Pitts at idle power?  Just wondering...

@quietflyer I haven't accepted any answers yet because I really am hoping for answers that talk about the Stearman, Pitts, and fighter planes.

Dive bombers, such as the Ju-87 and SBD Dauntless have purpose built dive brakes.
Aircraft designed with an emphasis low drag for fuel economy, such as airliners, are susceptible to rapid airspeed increase when put into a dive.
Extending flaps and spoilers can help control airspeed, but they too are limited to a maximum speed for safe deployment.
Biplanes are very popular for aerobatic flying because they are very strong and draggy.
So, there are 2 critical factors in dive recovery, airspeed and G force limits of the aircraft$^1$, and a third, proximity to earth.
$^1$ also of the pilot

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What is the difference between "x" and "+" configuration in missiles?
What is the difference between "x" and "+" configuration tail fins, and what are the advantages of each in free flight? (for example JDAM missiles)

The difference is a rotation by 45°.

@PeterKämpf you mean the max rotation of "x" configuration will be 45° ,or max rotation in "+" configuration will be 45°

@PeterKämpf: Very funny, but you're obviously missing something. The drawings imply that the tail fins operate differently (except for roll) in both configurations, so as to move the missile only in the vertical and horizontal axis in any of the configurations. So the question is valid if these different configurations do indeed exist.
OP: Do you have the source where the drawing is taken from?

@Scrontch: Missiles are stabilised by rolling. Yes, the effectiveness of the control surface varies slightly with roll angle, but in reality commands are almost always a mixture of pitch and yaw, and constantly changing due to the rolling motion. This is no either - or situation.

@PeterKämpf: Missiles are stabilised by rolling. Do you have any sources for that? From what i was able to pick up, i conclude this is NOT true. The vast majority of missiles (MANPADS are an exception) are roll-stabilized, i.e. they actively counter-act roll to maintain a fixed longitudinal plane. (Which of course can either be the + or the X plane, with respect to the control fins - which brings us back to the question)
cf.
https://maritime.org/doc/missile/part1.htm, section 5A2
https://books.google.fr/books?id=ubcczZUDCsMC, section 7-2

Mounting missiles below a wing or stocking them in weapon bays is much easier with X-Configured missiles. Look at these photos:
K-5M Missile Source
AIM-120 Missile Source
"+" mounted missiles would need more complex hard points and larger weapon bays.
Once fired the configuration doesn't matter anymore as all four control surfaces are continuously used to guide and stabilize the missile along its 3d trajectory.
If the missile only moves in the horizontal or vertical plane, the X configuration is better. For a fixed force F the angle of attack of the control surfaces ($\alpha_{cs}$) is smaller and can therefore be set faster, making the missile more agile. Assuming* a linear relationship between the the control surface force and its angle of attack:
$$\alpha_{csx} = \frac{F}{q \cdot S \cdot C_{L\alpha}}\cdot \frac{\sqrt{2}}{4}$$
$$\alpha_{cs+} = \frac{F}{q \cdot S \cdot C_{L\alpha}}\cdot \frac{1}{2}$$
For a quadratic relationship* between the drag ($D_{cs}$) and the angle of attack of the control surfaces there is no difference in the total drag.
$$D_{cs+} = q \cdot S \cdot (4\cdot C_{D0}+2 \cdot C_{D\alpha} \alpha_{cs+}^2) = 4\cdot q \cdot S \cdot C_{D0} + \frac{1}{2} C_{D\alpha} \cdot (\frac{F}{q \cdot S \cdot C_{L\alpha}})^2$$
$$D_{csx} = q \cdot S \cdot (4\cdot C_{D0}+4 \cdot C_{D\alpha} \alpha_{csx}^2) = 4\cdot q \cdot S \cdot C_{D0} + \frac{1}{2} C_{D\alpha} \cdot (\frac{F}{q \cdot S \cdot C_{L\alpha}})^2$$
*If someone has better assumptions for lift and drag please comment or correct the post.

Note that some missiles, such as the AIM-7 Sparrow use the upper fin for mounting. They have moved away from that, though. Maybe due to occurrences like this

@TomMcW Thanks for the link! I had looked for + mounting examples but could't find any.

@TomMcW I'm loving that link

also keep in mind that the weapon may execute a 45 degree roll after release, turning one option into the other in flight.

@jwenting i appreciate your help, but which one do you prefer for maneuvering "x" or "+" and why ?

@jwenting Yes, that's why I wrote that once fired the configuration doesn't matters anymore: it will fly following some 3d-trajectory combining pitch, yaw and roll movements in unknown amounts depending on the target (airplane) and disturbances.

@Michael I've not done any study as to the advantages of either in specific scenarios (which would be an issue, I doubt one or the other is universally superior). I was just commenting on the fact that you can't determine the flight profile based on the way the weapon is attached to the carrier aircraft.

@TomMcW great story on the link!

@Gypaets i am wondering what is the best equation for this kind of missiles that has no engine like JDAM  , is gliding angle the best one for this kind of fall , links is below    https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/glidvec.html               https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/glidprob.html

The difference between cross and plus configuration has nothing to do with equations of motion.
The difference is just in control system implementation and transferring the control surface deflections to equivalent 3 plane surfaces.
for example cross config can be controlled better, while plus config in simpler.
or:
for optimum control efort for a specified da dr de:
in plus: d1= da-dr      in cross:d1=da+de-dr

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Can I have a free ride with airplane?
If I want to go from A to B, and there happens an airplane also goes from A to B, which has an empty seat, can I ask to go with it? It's like having a free ride in a carpool, but with airplanes?
I know this is ridiculous, but it will be an rewarding experience to think of. I'll be able to learn a lot. I was once almost too late to board, and was transported to the plane by a staff car, not by regular bus. I guess that I could have broken many safety rules, but if I can safely go with that car instead of the regular route, I would do it again.
If yes, what kind of airplane I can travel with? I guess in a commercial plane, I am expected to pay the ticket, so there's no way for me to go? A cargo plane might not be designed for an untrained person, but I guess as long as I don't lose consciousness, then it's still considered safe? A jet fighter... sorry, never mind. If no, what reason would it be? Is it because of safety, liability, or something else?
I will have one luggage and one backpack with me. Obviously I won't pull the exit door when the plane in the middle of the air.

Near duplicate: https://aviation.stackexchange.com/q/2045/167

Before downvoting, remember that ignorance is what we are here to fix.

Another near duplicate

You could probably get a good price for a flight if you offer petrol and landing fees. For a pilot who is building hours, this may be acceptable.

@mins it's (1). I've updated my question, do you see it clearer?

@mins yes, that's what I mean. The logic in here is that I would like to try out different experience. Being a airplane hitchhiker is one kind of that.

Just out of curiousity, why do you think carpooling is "free"?  From what I've seen, either the drivers take turns, or the non-driving riders pay for gas.  You can probably find light plane owners who'd take you for a ride on similar terms.  I've done it myself, when I and others wanted to go places with no/awkward commercial flights.

@jamesqf inn my experience in my country, the price for carpooling is cheap enough that it can be considered as free. So it is possible to have "plane-pooling"?

@Ooker: Perhaps carpooling means something different in your country.  Here it's a voluntary arrangement between individuals, so there's no set price.  But it's hard to see why such groups wouldn't try to share costs equally.  As for "plane-pooling", that's essentially what I did e.g. taking a couple of colleagues to a conference in my plane.  If several people had to make the same trip regularly, sure, as long as applicable FAA (or equivalent body) regs are followed.

Can you have a free ride in an airplane?
Yes, probably!
Pilots give free rides in airplanes all the time. They're generally called discovery flights, and (at least in the USA) you can find someone who offers them at most smaller airports.
Pilots are friendly and we love to introduce new people to the world of aviation. That said, flying is expensive, so you may need to pay for the cost of the airplane/fuel, if the pilot is making a special flight just for you.
Can you have a free ride in a commercial airliner or scheduled air carrier?

You will almost certainly not receive a free ride in a commercial airliner or scheduled cargo aircraft.  Think about it: if flying for free was an option, why would anyone ever pay for a ticket? Also, again in the USA, the TSA will have serious problems with individuals who have no ticket for a flight being on that flight. Cargo carriers will be the same.
What about unscheduled (air taxi) operations?
Your chances are better!  See this answer by casey for whether passengers can ride in the jumpseat in the cockpit of a 135 flight. Bear in mind that you still need to be legally present on the flight, which means that the pilot/company needs to permit you onboard.
What about a fighter jet?
Sure!  The Blue Angels regularly take journalists for rides. Contact the US Navy or US Air Force public affairs office when the Blue Angels or the Thunderbirds are in your town.
Why are you asking about bringing baggage?
Demo flights don't work the way commercial flights do. Asking to bring along luggage indicates that you are either ignorant or you think you're entitled to fly for free. You are not.  If you are given the opportunity for a demo ride it is beacuse of the generosity of the operator, which you are taking advantage of.  (You will probably receive downvotes on this question because you don't understand this.)

As I understand it, the Blue Angels usually give flights to a local journalist or two for PR purposes. There's no harm in asking, but I'd bet they are rather unlikely to pick you unless you can offer them significant press coverage or otherwise contribute something that helps them, say if you're the mayor or something.

Can you give more explanation on why commercial airliner or scheduled air carriers cannot bring me in? Maybe the safety rules or the procedure I guess? As for the ticket, traveling for free is actually an option if you really want, yet people still happily pay for it.

@Ooker It's not a safety aspect, the airline assumes some liability when you get on board, liability that could cost them greatly in the event of an accident. They typically won't assume this liability for free.

I haven't ever heard of a free Discovery flight... Young eagles offers free flights to youth.

”you may need to pay for the cost of the airplane/fuel, if the pilot is making a special flight just for you" a private pilot can't do that, it would be considered "holding out". You could be required to pay a share of the costs but even that has to pass a pretty strict set of requirements.

@Ooker - Can you give more explanation on why commercial airliner or scheduled air carriers cannot bring me in? It's not that they cannot, it's that they choose not to. The question you must answer is "What makes your desires so much more important than everyone else's that you deserve a free ticket?"

@Steve V.: Well, some airlines do sort of give away "free" flights, if you have say a credit card that gives frequent flier miles instead of cash back.

Not mentioned:  Discovery Flights and most any free ride flight will land at the same airport it departed from.   Trying to bring luggage or make distance on a journey will be fruitless.

@RonBeyer your comment really explains a lot. If you can make it to a full answer, I'll accept it.

@jamesqf Only to the extent that cash-back is "free money". It's paid by someone, somewhere. Credit card companies aren't in the business of charity, and (I'm pretty sure) neither are airlines.

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How and why do wake vortices come together to form two tip vortices?
Wake vortices are created all along the wingspan but, the vortices come together to form two tip vortices. How exacly does this happen and why does it? I know that wake vortices push air to the side. Does this sideways moving air somehow pull the vortices to the tip?
Here is a picture of what I am talking about:

This picture shows that there is downwash and vortices along the whole wing span but, for some reason there are only two vortices at the tips.
One theory I have is that the up wash from the vortices follows the downwash from the rest of the wing and they spin around in a circle but, I am not completely sure

The roll-up at the tip is because the tip is a discontinuity between air accelerated downwards, and free air stream. There are two excellent answers about tip roll-up at this question.

The picture in your question is an oversimplification. Maybe this photo of an MD-11 on a moist day will help to illustrate the point:

I also think you should not call the wake vortices "tip vortices". The tip vortices are an insignificant part of the whole wake system and can be seen in the picture above on both the winglets and the horizontal tail as thin lines of condensation. The wake vortices are the result of the vorticity shed over the wingspan rolling up. With flaps set for landing, the MD-11 has a big change in circulation at the flap tips, so here the shedding is intensified and shows as a thick line of condensation. There is more going on left and right of that thick line, but it is not visible because the gradient of circulation over span is too shallow to cause condensation in the wake (it does over the wing itself, due to the low pressure on the top side).
Maybe it will be best for understanding wake rollup if you watch it happen in a video. It should also help to confirm your theory :-). A good one is at 3:20 and again at 5:40. Also, watch how the wake changes when ground effect kicks in at around 4:00.

how looks air in wake between wingtips, without flap,is air straight or full of vortices?theory predict trailing edge vortices but where they are?

@JurgenM Yes, there is a sheet of vortices being shed, so in theory there are infinitely many. But their velocities all add up so you have consistent downwash behind and upwash at the sides of the wing. Remember my advice that it is best to forget all that potential flow theory because it is not helpful in understanding what really happens?

If their velocites all add up, that mean they dont exist in reality,can not be seen? Can you give answer here, becuase some members tell they exist and can be seen? https://aviation.stackexchange.com/questions/95899/does-trailing-edge-voritces-exist-in-reality

You have to think of where pressure is lowest and where it is highest.

The wing is forcing air downward along its entire span, creating a low pressure area above the wing and high pressure below it. Higher pressure air wants to move in to fill the low pressure voids. The ambient air above and away from the wing moves downward toward the low pressure zone. This accelerating air causes lift, and the downwash.
At the wingtips, in addition to ambient air above and below, the accelerating air encounters ambient air to the outside, so the ambient air moves inward toward the low pressure zone. The high pressure below the wing pushes outward toward the ambient air. This causes a rotation. The rotation is the beginning of the vortex. It begins at the wingtips because that's the first place it can occur.
As the wings move away they leave a sheet of low pressure behind them. The low pressure zone is pulling ambient air downward and inward from above whereas the high pressure is pushing downward and outward into the ambient air. As each parcel of air moves toward the voids, the parcel behind it moves in to fill in where it was, so the whole process expands outward as it involves a larger area. The rotation that began at the wingtips spreads outward creating the rotating funnel shape.
You can see this in the photo below. As the vortices spread and the moisture of the engine contrails get caught in them you can see how they move. The inner contrails are being pushed down and out. At the top the contrail the parcels of air are being pulled in and down toward the void and the outside is being pulled up and around to fill in where the first parcels left. The sheet in the center is the low pressure area. The low pressure is causing water to condense into a fine mist that can be seen by the sheen near the top of the photo.

Zooming in on the wingtip you can faintly see the vortex forming at the tips.

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Helicopter crop duster minimum altitude over homes (not field)
I am asking specifically regarding helicopter making approach to  field over home adjacent to field being treated.
In this example:
Helicopter repeatedly passed over my single story rooftop at easily  15-30 meters above rooftop, or less.
Spray contacting me at same time.
Is this allowed, especially when there appear to be 2-3 other possible alternative approaches that would not require flying over the rooftop of a single family dwelling at such a low altitude?!

Thank you so much for your directions to previously posted information. I am appreciative. I'm truly wanting to understand regulations or generally accepted practices in effort to have a constructive dialog with crop dusting company.     All the best.


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Why can't we think of mechanism to save people in case of airplane problem?
Given it is 21st century, why we can't think of mechanism which will save people in case there is problem with the airplane during flight? e.g. so that cases like MH370 don't happen?
Is this really such a difficult engineering problem?

I'm sure we could if we were willing to spend 100 times more on tickets and have all of the versatility taken out of it.

What do you mean? Lives are saved every day because such safety mechanisms already exist.

Why can't we think of mechanism to save people in case of insert random human activity here?

BTW. Every time I fly, before takeoff flight attendants perform obligatory lecture about "mechanisms that save people in case of airplane problem?" - oxygen masks, life rafts, emergency exits etc...

This question and this one might be useful.

There are, as many others pointed out, many thousands of systems, saving the aircraft.
But I assume you mean things like ejection seats or parachutes for passengers:
There is indeed already a mechanism to save the passengers of a plane. Not for airliners, but for general aviation planes. It's called the Ballistic Recovery System (BRS). Cirrus built it into their Cirrus Aircraft and named it Cirrus Airframe Parachute System (CAPS).

Source

There are millions of safety mechanisms in aviation and they save lives every day.

You are right of course.


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Why is there no numbering system for more than three parallel runways?
If an airport has three parallel runways - a good example being Schiphol (EHAM/AMS) - the parallels are usually labeled "left", "center", and "right", and in case of Schiphol they are 18/36 L/C/R.
However, if there are four or more parallel runways, one or more have to be given a different number. For instance, ORD has five parallel runways heading 93.6-273.6 and their designations are 9/27 L/R and 10/28 L/C/R.
Why couldn't these runways all have the same number and add a second letter - I, M, or O (for Inboard, Midboard, or Outboard) so the number can accurately indicate the runway's heading? Under this system, ORD's 10R/28L could become 9LO/27RO.
Is it because the I and O could easily be mistaken for a 1 and 0?

The heading will be inaccurate for a lot of airports (generally, and in the near future specifically) anyway, so being plus-minus one point off isn't that great a deal, unless you also mandate perfect accuracy in what is actually a rounding error!

"Because those are rules."
ICAO Annex 14 (volume 1) provides the following guidance:

5.2.2.4 A runway designation marking shall consist of a two-digit number and on parallel runways shall be supplemented with a letter. On a single runway, dual parallel runways and triple parallel runways the two-digit number shall be the whole number nearest the one-tenth of the magnetic North when viewed from the direction of approach. On four or more parallel runways, one set of adjacent runways shall be numbered to the nearest one-tenth magnetic azimuth and the other set of adjacent runways numbered to the next nearest one-tenth of the magnetic azimuth. When the above rule would give a single digit number, it shall be preceded by a zero.

and

5.2.2.5 In the case of parallel runways, each runway designation number shall be supplemented by a letter as follows, in the order shown from left to right when viewed from the direction of approach:

for two parallel runways: “L” “R”;
for three parallel runways: “L” “C” “R”;
for four parallel runways: “L” “R” “L” “R”;
for five parallel runways: “L” “C” “R” “L” “R” or “L” “R” “L” “C” “R”; and
for six parallel runways: “L” “C” “R” “L” “C” “R”.

Presumably, the rules for runway designation were created before anyone had the fantasy to imagine mega airports with 4 or 5 parallel runways like we have today. They have then later been adopted to accommodate such airports, without changing the core principles, since doing so would potentially require thousands of airports worldwide to rename their runways and change associated procedures. Not to mention the changes that would be required to aircraft and ATC software, meteorological reporting formats, training and certification of various personnel and so on. Simply put, the disadvantages of using a slightly flawed system are significantly smaller than the cost of redesigning it completely.

There are only a handful of airports with 4+ parallel runways, and it's simpler to just accept them as exceptions than to add complexity to a naming system that works just fine for thousands of other airports around the world.

(Short) list of airports where this matters here.


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What vertical separation is required in a holding stack to avoid wake turbulence?
Sometimes in my free time I play Endless ATC, a game where one acts as the approach and departure controller. In the game (as in real life) the controller is forced to put planes in a holding stack, separated vertically by 1000 feet.
Is this vertical separation - 1000 feet - enough on its own to prevent one plane's wake turbulence from affecting the plane immediately below? Or is it possible that I've accidentally had planes fly in each other's wake turbulence?
If so/if not, then what's the minimum vertical separation necessary to ensure one plane's wake turbulence doesn't affect the next plane in the holding stack?

The minimum vertical separation in a holding stack is no different from the minimal vertical separation used elsewhere. This is almost universally 1000 feet. In countries where Reduced Vertical Separation Minima (RVSM) has not been implemented, 2000 feet of vertical separation is required above FL290. 2000 feet is also needed if a non-RVSM equipped aircraft (such as certain older military planes) is flying above FL290. However, not many holding stacks grow to above FL290.
A point to consider is the fact that aircraft produce the most wake turbulence when they are flying slowly for landing. So since 1000 feet is enough during approach, it should also be plenty in a holding stack.
There has been some evidence that 1000 feet of vertical separation may not be enough for certain, heavy aircraft types. In 2017, a Challenger jet had an accident after passing 1000 feet below an Airbus A380 during cruise, presumably because the smaller business jet hit the wake turbulence from the A380. See AVHerald for details. However, this has not (yet) resulted in changed separation minima.
1000 feet is not enough to prevent planes from "flying in each other's wake turbulence" as you say, but in most cases it should be enough to ensure that the wake turbulence has weakened enough to not pose any threat to the lower aircraft. Also remember that wind can have a significant effect on the spread and dissipation of wake turbulence, so on days with a certain wind direction and velocity, the wake turbulence may survive longer than other days.
But the answer to your question remains the same: the vertical separation to be applied in a holding stack (below FL290 and during RVSM) is 1000 feet.

The minimum separation in a holding stack is still 1.000ft. Given that wake turbulence vortices move with the wind direction, you would have to fly the holding pattern with a direct headwind or tailwind to hit one. Or your speed difference between two aircraft in the same pattern by pure chance correlates to the wind speed and direction. If you have enough space, you can surely work with 2.000ft, but 1.000ft will remain the minimum separation.

Not doubting you, just wondering if "The minimum separation in a holding stack is still 1.000ft." is jurisdiction specific?

As far as I know, the 1.000ft are ICAO specific, so it should apply across all jurisdiction that operate under ICAO. At least I have not heard of any other country using less than 1.000ft in non-RVSM airspace, let alone in RVSM Airspace. https://www.icao.int/safety/fsix/Library/AppendixG_to_Part-91.pdf

From some of the ATC feeds I've seen transcribed controllers seem to prefer 2000 feet separation.
Which makes sense if you consider that airplanes aren't going to hold on the exact altitude (which would then lead to violation of separation) and think about what can happen to airplanes in a holding stack.
Using 2000' separation also lets the controller move the entire stack down 1000' in a single series of commands instead of needing to wait until each aircraft has descended to its new altitude.

Downvoted because this answer suggests 2000' separation is needed, which is factually incorrect.


text:
Was this a near collision or a normal procedure?
I flew on a plane from UK to Spain a couple of days ago (FR7542, 15/01/19 16:30) and in the middle of the flight I could see another plane (a private jet much smaller) just flying underneath us.
From my point of view it was excessively close. And given that I came across recently an article in the news saying that two planes of the company I was flying with flew a hundred meters close to another aircraft of the same company. I was a bit worried for that. Just wanted to share this here and hopefully get some explanations.

Can you provide any other info? Flight no., date, time, etc.? These things always turn out to be farther away than you think they are

Hi @TomMcW, It was FR7542. 15/01/19 16:30

There is only 1000 ft of separation between levels of traffic at the altitudes jets operate in (above 29000 ft) and when you see someone 1000 ft above or below it looks really close.  When you are sitting in the flight deck the first time on a busy airway and oncoming traffic passes above or below, you think they are passing only a few hundred feet away even though they are 1000 ft.  When you first notice an oncoming contrail, it looks like it's coming straight at you until it's a mile or two away.
The Traffic Collision and Avoidance System (TCAS) on each airplane tells the crew the proximity and relative altitude of nearby traffic and the system gives warnings with directions to climb or descend if someone is on a collision path. Plus ATC knows everybody's altitude and the ATC computers provide warnings to controllers if an airplane deviates from an assigned altitude by more than a couple hundred feet.
It takes a lot to go wrong for airplanes to come too close to each other, although nothing is perfect and it can still happen.  The systems in place keep the probability to an acceptably low level.


text:
Where can I find US ARTCC sector boundary coordinates?
For an academic purpose, I would like to map US airspace using GIS tools with coordinates. I found the airspace boundary coordinates from the eNASR, but no luck with the sector boundaries.
Any information you can provide me would be greatly appreciated.

The center boundaries are available online but sector boundaries are hard to come by. I expect it's because they change often and are really not all that useful to pilots.

Users of virtual ATC networks will probably have the answer (VATSIM, IVAO) as they need to define their sectors as well.

For any ARTCC you can contact their Airspace and Procedures office. Be polite, and specific. They will have that information. It should be publicly available. The cities for the lower 48 where ARTCCs are (associated city name and actual location)
Albuquerque (NE side of town) 505-856-4500
Atlanta (Hampton GA you should hear it when the speedway fires up) 770-210-7600
Boston (Nashua NH) 603-879-6633
Chicago (Aurora IL) 630-906-8200
Cleveland (Oberlin OH) 440-774-0300
Denver (Longmont, my facility, tell 'em CD from training sent ya) 303-651-4100
Fort Worth (north side of DFW airport) 866-835-5322
Houston (just south of airport) 281-230-5600
Indianapolis (at the airport) 317-247-2234
Jacksonville (Hillyard FL) 904-845-1500
Kansas City (Olathe) 913-254-8400
Los Angeles (Palmdale) 510-745-3000
Memphis (NE of airport) 901-368-8101
Miami (North Miami in a warehouse district...) 305-716-1500
Minneapolis (Farmington MN) 651-463-5514
New York (Ronkonkoma) 631-468-1001
Oakland (Fremont) 510-745-3000
Salt Lake City (airport) 801-575-2400
Seattle (Everett WA) 866-835-5322
Washington DC (Leesburg VA)703-377-3401
These numbers are for their respective switchboards, select "Airspace and Procedures" from the menu.


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How does HSI decide to give a left or right CDI indication?
Since there is no reverse sensing in HSI, it tells you to turn left or right,
So Is the interpretation correct that it will tell you to turn to the direction to that direction, which requires less turning - to intercept that radial.?
For example,
Our present radial is 070 degrees,
course selected is 90 degrees ,
So if if the heading is 355 degrees, would it tell to turn left since it requires less turning ?

AFAIK, the HSI does not tell you which direction to turn. Do you maybe mean a flight director instead?

First, an ordinary HSI doesn’t contain digital logic to decide anything.  Unlike an FMS or flight director,  it no more tells a pilot where to go than a $5 compass recommends a direction to a disoriented hiker.  Both simply present a display that informs the user of the situation.  Even the full name - Horizontal Situation Indicator - implies those same limitations.
Next, if you think that in your scenario a left turn is the best method to get established on an easterly course of 090 from a navaid, then I propose we have a friendly imaginary race to demonstrate why that might not be the best choice.
We will run 2 heats.  You will turn left while I turn right.  We agree to perform standard rate turns, and fly at precisely the same airspeed.
Here are the objectives:

Heat 1:  Be the first to cross the 090 radial.
Heat 2:  Be the first to reach the 090 at 20 DME.  (I must make the initial turn at least 180 degrees to mirror your turn)

I will describe the results, and offer an illustration.  Ready?  OK, here goes…
Heat #1:  You have already figured yourself correctly to be the winner because you have less turn to perform.  Not 5 degrees less, but 10 degrees – you turn 175 degrees left to 180, I turn 185 degrees right to the same heading.  Your margin of victory, however, is miniscule.  A standard rate turn is 3 degrees per second, so I would spend just over three seconds more than you turning.  But because we are heading in generally the same direction as we both initiate and complete our turns, the actual margin is much less.  (the angular change doesn’t equal forward speed)  Anyway, the victory is well within the margin of error and exceeds either of our abilities to control flight parameters well enough for consistent results.  (as well as my ability to actually compute it!)
Heat #2:  I’m providing an illustration for this one because this is where it really makes a difference.  Because our objective is never just to get to a radial, we must consider our actions in the context of where we intend to go after we intercept.  As you can see, I beat you soundly by a margin of 4 times the radius of turn.  We could calculate a distance, but we don’t need to – at a standard rate of turn I will have beaten you by 2 full minutes.
Note:  I just realized that 4xRadius is correct, but not 2 minutes.  Because the 2 minutes spent turning in opposite directions was along an arc.  Pi is probably needed here, but it’s getting too late to think on it and make a correction right now.

Reality:  I would either make a right turn and proceed direct to the fix, or set up for an angle that will get me on the radial expeditiously, while not sacrificing downrange travel towards the destination.  Because the real goal is to get to your destination in the most direct and efficient manner, you should generally always plan a turn in the direction of travel.  Intercepting a radial is just one means to the end, and turning the opposite way to save a few seconds of turn will cost you way more in distance traveled in the wrong direction.
Angles between 20 and 45 degrees seem to be the best compromise for intercepting.

HSI with regards to VOR information is a positional instrument so you can strike "heading" out of any question. For the given position on the 070 radial with an OBS setting of 090 then you will have a CDI deflection RIGHT twenty degrees and a FROM flag shown. This is the displayed state regardless of the heading of the airplane. Be aware that because an HSI will spin in the dashboard according to airplane heading. For example if heading is 270 with an OBS setting of 090 the HSI will be displayed upside down. The rightward CDI deflection will be toward the left wingtip but is still considered a right deflection.
Considering all possible positions given an OBS setting of 090 the CDI will have the following indications. For positions north of the 090 radial (this line running east-west through the station), the CDI will deflect right. You can imagine the CDI as the "radial road" which would be to the right of someone traveling a course 090. You'd be parallel to the desired radial displaced north (left) of it.
For positions east of the station (all radials from 360 through 180) the flag would show FROM. This can be thought of as any motion along the OBS 090 course would be increasing distance from the station.
The deflection can be anticipated by answering the question: "From a particular position, facing the azimuth of the set OBS, would the selected radial by right or left?" The TO/FROM flag can be anticipated by answering the question: "From a particular position, if I traveled in the direction of the set OBS, would I be decreasing or increasing distance to the station?"


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Can a turbofan run without the fan to provide power without thrust?
Is it possible for a turbofan jet - like on a 747 - to run without the fan, if for example you just want the thrust from the internal jet to use to heat up water, not to use as thrust?

Sounds about like an APU... electricity + bleed air, but no thrust.

Tangential: companies like General Electric use/modify the gas generator core of aviation turbine engines for use as terrestrial or marine power generators. An example would be the LM2500, which was derived from the CF6 turbofan.

To clarify - are you asking if the fan can be braked or decoupled (leaving the core to run alone), or if cores can be re-purposed to work without fans?

If you just took away the fan, there'd still be thrust (from the core), and you'd have a turbojet.

So... hotel mode for turbofan.

What @Sean said.  A turbofan without a fan is essentially a turbo jet. I'm sure there is documentation that explains what % of a turbofans thrust comes from the fan and what % comes from the combustion gas.

No, the fan is directly connected to the outer compressor and turbine (N1). There is no direct connection to the inner compressor and turbine (N2), which could run independently, but there is no switch to somehow stop the N1 rotation in a 747 (and I am not sure, if it would run properly anyway).
Some turboprops (e.g. ATR-72) can run an engine without the propeller (Hotel Mode, see e.g. this question), which is connected to the engine via a gearbox. While there are also geared turbofans, you cannot completely disconnect the gearbox from the core engine.
As the other answers have said, it is however possible to build a jet engine without a fan in the first place. The Boeing 747 even has one in the tail: the Auxiliary Power Unit (APU), which will provide electricity and bleed air while the main engines are not running.

The only correct answer so far - but it would be useful to mention that this operating mode of turboprops is known as "hotel mode". And it's loud and inefficient compared to an APU.

@pericynthion Thanks, I added a link to the relevant question.

Yes, it is possible, in fact it is pretty common.
It is called a turboshaft engine or gas turbine. You can find examples of their use in:
APUs
An acronym for Auxiliary Power Units, these are probably the closest to what you have in mind, this is a jet engine, usually quite small, that is used to provide onboard power and sometimes hydraulic pressure. You can often find these located in the tail cone of modern airliners, with the exhaust at the cusp of the cone.
Helicopter engines
Most helicopters worldwide are powered by this form of jet engine, optimized for mechanical output, since helicopters generally do not benefit from exhaust jets. Otherwise it is very much a jet engine that drives a helicopter rotor (via multiple gearboxes to reduce rpm) instead of a fan.
Tank engines
Just like the helicopter version, although with different optimizations. You can find these in the M1 Abrams and the T-80, among others.
Electrical power plants
The generation process at most modern gas-fired electrical plants involves a gas turbine that works like an upscaled turboshaft, although usually in combination with other systems.

Would the downvoters explain their reasoning? If it is because the OP´s question can be interpreted literally as taking an existing turbofan and using it in hotel mode, then I have to point out the OP also asks about using "thrust from the internal jet to use to heat up water", so I feel the reinterpretation is a bit too liberal.

Yes.  A turbofan without the fan would be called a turboshaft engine.


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Where can I find approved entry/exit points for Free Route Airspace (FRA)?
How can I find approved entry/exit points for Free Route Airspace (FRA) in different countries?


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Why does Boeing 247D have reverse slope windows when they were removed from the D models?
Have a look at this article on a famed example of the 247D, DZ203, the first aircraft to make a completely blind landing. The first image is the aircraft during its flights with the RAF, which places it after 1942. Note the reverse-slope windows that are a hallmark of the early 247's.
However, this is a D model, and the D model of the 247 is noted as having had these removed and replaced with normal slope-down windows. This can be seen in the images of the other 247Ds on the same page, which were produced in the same batch of aircraft.
Does anyone know what's going on here? Were certain batches one way or the other?
NOTE: No, this is NOT a dup. The original answer from a while back states that all 247D's had forward slope windows, but this example clearly does not.

Does this answer your question? Why do early airliners have "backward slope" windows. Basically it just creates more glare not less.

The Boeing 247D, DZ203 has construction number 1726 and was built in 1933.
The first production Boeing 247D produced made its first flight on September 5. 1934 and was scheduled to be delivered to United Air Lines late September 1934. (It didn't enter service with United immediately, instead it participated in the 1934 MacRobertson Race first, where it finished third.)
So DZ203 was built as a standard Boeing 247, and then later converted to the 247D standard. In the conversion the engines, cowlings and propellers were changed, but the windshield remained in its original style. This was common for 247D conversions.

Excellent, thank you!

Makes sense, I would think that changing the slope of the windshield would be a very expensive airframe modification.


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E-2/C-2 left thumb control?
For some reason I ended up binge watching E-2s and C-2s doing shots and traps on YT. One thing caught my eye; especially during the approach, the pilot’s left thumb is in constant motion pressing a control on the yolk.
One imagines the radio PTT, but there seems to be no relationship between the clicks and talking during the times where both are visible, and it’s not like they are push-n-holding, it’s click click click!
So then one starts thinking about trim, but what madman designs a plane that requires so many trim adjustments during landing, and then requires a separate push for each adjustment?
Anyone know what’s going on? Here’s an example:

UPDATE: here's another example:

Look around 0:55 and again at 1:25 - his thumb is positively dancing. I can see in this video it is a hat, so I guess it is trim.
Never having flown anything larger than a s-twotter, what is going on here? I'm used to adjusting trim maybe one or two times in the circuit, this is nuts.
P.s. why the heck is there a privacy screen on the door?

Possibly related to the "direct lift control" system?

The "privacy screen" is probably more about blocking light during night flying.

Don't see what you're referring to in that video, but it's almost certainly elevator trim.

Yeah, you didn't pick a good example video.  And I agree with Ralph, probably trimming...

It would be good to find out how the direct lift control system is controlled on this aircraft and find out whether or not the thumb button has anything to do with it.

“but what madman designs a plane that requires so many trim adjustments during landing”—it does not depend on the plane but on the speed profile. Trim corresponds to speed, so it needs to be adjusted as the plane slows down. And they seem to do most of the slowing in the pattern.

Most likely the pilot is trimming the airplane to alleviate control pressures after he adjusts his airspeed in the pattern.  Ideally you want to trim out any kind of control pressure that you have to place on the yoke, or rudder pedals, first adjusting your speed as desired using the power levers, then trimming using elevator-rudder-aileron trimming procedure. The “coolie hat” switch on the left grip on the yoke provides both elevator and aileron trim.

Privacy screen is on the D.. and it's for light discipline in the back of the plane for the operators.  There are three people sitting back there with very little light while the systems are in operation, when landing, or taking off the operators are sitting facing forward, lights on, most systems are off.
There are other reasons in the past, for C and earlier models that don't really apply to the D.

"it's for light discipline in the back of the plane" - ahhh, yes.

The left coolie hat provides elevator and rudder trim. Coming into the break at 250 KIAS (or greater), there is quite a bit of deceleration to get to “on speed.” Typically this would require quite a bit of nose up trim inputs. The rudder input would vary based on the idle power setting (in the break), slowing down, and adding power back on (to catch “on speed”).
As an example, I would roll into final with a slightly fast speed, then click 2 times for nose up trim while pulling power / doing the wings level transition into the groove. This would keep the plane on speed as I added power back on the aircraft to maintain the glideslope.


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What are Altitude, Attitude and Heading Hold?
Can someone give a good and simple explanation about these three dynamics?

Altitude Hold
Attitude Hold
Heading Hold

These are generally 3 different auto pilot modes. The first two are related to pitch and the third to heading/roll a nice overview can be found here
Altitude Hold: Generally speaking setting an autopilot to altitude hold will cause the autopilot to maintain that altitude by varying the pitch of the aircraft. Depending on the system it may attempt to maintain the altitude even its not possible which can lead to a dangerous situation.
Attitude Hold: Is the autopilot setting that will hold the pitch of the aircraft constant when set to this mode. For example if you depart and climb out at an 8 degree nose up attitude, engage attitude hold, the aircraft will hold 8 degrees nose up until set otherwise or shut off. As noted in the comments this can create a very dangerous situation as the aircraft may continue to maintain pitch (generally for a climb) even if power does not allow this can cause a stall at high altitude. More sophisticated auto pilots allow for constant airspeed climbs which are far safer.
In my experience you are generally in either altitude or attitude mode at any given time.
Heading Hold: This is a roll control mode that holds the heading of the aircraft (keeps it on track). It will typically be slaved to a bug on the HSI where you can set the heading the autopilot flies.

Note that attitude hold is even more prone to inducing dangerous situations than altitude hold, especially in climb. Preferred mode for climb is “open climb”, which maintains speed with pitch.

As long as you are mentioning airspeed mode, vertical speed mode is also common as well.  More detailed info is available here: Flight Level Change or Vertical Speed - When to use it?

There is also GPSS, or GPS Steering, where an in-panel (typically) GPS or GPS Moving Map, controls the autopilot steering. In my plane, the GPS 'talks' to the DG/HSI which then connects to the autopilot. The DG/HSI is used to select between Heading and GPSS.

@MichaelKjörling :-) Yeah, or maybe decreasing!

@Richard Even better.

We use FLC (pronounced feltch) mode for climbs (constant airspeed but varying (usually decreasing) verticle speed) as opposed to VS mode (constant vertical speed but varying (usually decreasing) airspeed) for climbs in jets. Failure to do so can (and has) led to autopilot induced stalls.


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Why do we have to check that the fuel x feed is properly functioning before departing for an ETOPS flight on the A320?
In the A320 family series FCOM under the special operations procedure chapter it is mentioned that we have to check the fuel x feed on ground before departing.
Any specific reason why we only check it before departing for ETOPS flights?

The main reason to check the Fuel-X-FEED on the ground before the flight is to allow the pilots to use the total fuel quantity on both wings in case of a single-engine failure during flying in ETOPS.
(Due to engine-out scenario you may not be able to feed one engine by both fuel tanks without the FUEL-X-FEED Valve)

in addition to this, the Fuel-X-FEED in the A320 MEL is a NO DISPATCH item by means you can't depart without and that reason tracks back to the EASA Requirments listed in AMC-20 AMENDMENT  - 7 (APPENDIX 4 - FLIGHT PREPARATION AND IN-FLIGHT PROCEDURES):

(11)  Fuel  system
(i)    The aeroplane fuel system should provide fuel pressure and flow to the engine(s) in accordance  with  CS  25.951  and  25.955  for  any  fuel  pump  power  supply  failure  condition not shown to be extremely improbable.
(ii)   The fuel necessary to complete the ETOPS mission or during a diversion should be available  to  the  operating  engine(s)  under  any  failure  condition,  other  then  fuel  boost pump failures, not shown to be extremely improbable2 (e.g. crossfeed valve failures, automatic fuel management system failures).

@Islan Almaghraby if that's the case shouldn't be checking it before every flight ? Do you have any information or official reference which explains the reason specifically for ETOPS?

@Jai - On a long overwater flight you will have a greater need for crossfeed capability if you lose an engine and still need to travel a long distance than if you are overland with multiple divert options. So it's not a bad idea to check every time, but I could understand not requiring it.
P.S.  I notice that you use the term "we" as if you work on or fly these aircraft. Wouldn’t it make sense to ask the people you work with rather than polling random people here?

@Micheal Hall appreciate your input I didnt mean any offence. I am a professional pilot and I do fly the A320 for a living. Alot of the questions I post on this forum the answers aren't available on the internet. It does make sense to ask my airline especially my engineering department and I do ask them as and when time permits me a number of questions have been answered by them but alot still haven't as I cant always ask them due to circumstances at hand.

@Islam Elsayed I checked the document which you listed but couldn't find a definitive answer for it.

@Jai You may find it on page 14, paragraph 11.

@IslamElsayed I added the text from the document rather than an image. We prefer that since it's searchable like this.

Thanks, @Bianfable appreciate your input always!

@Jai since this seems to be pretty conclusive information, do you want to give it a check-mark so others will know that it's been satisfactorily answered? If not, you may want to clarify what point(s) you're still questioning.

@FreeMan done


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What is it called when you roll without yawing or pitching?
My instructor made me do an exercise that consists of banking the sailplane left and right around 30 degrees without moving from the axis. I need to aim for a specific point, and start with the exercise. This is a coordination exercise but does it have a name? It's like a dutch roll but I need to stay in the roll axis without moving.

I know it's an old question, and you'd have to look at other answers and see if they might invalidated by a change, but could you clarify either as a comment or an edit, whether you were supposed to keep the yaw string centered, or whether you were supposed to make rudder inputs to help hold the nose on the point even if they made yaw string go off center?  Thanks.

The string, if you do it perfectly, should be centered because you are perfectly coordinated, but as a student, I needed to add some rudder inputs in order to keep it kind of centered. You should move the stick and rudder all to one side, and when is banked enough, all the way the other side in one continuous movement. And so on, the string shouldn't move

In the aerobatic community we would call that the beginning of an Aileron Roll.
It's harder than it looks.

But "Do a barrel roll" sounds more fun than "Do an aileron roll"

@workoverflow True, but a barrel roll is a different maneuver with significant change in pitch and altitude.

@workoverflow I get it. :P I saw the title and I was like "Wait a second, I know this... thanks, Game Theory!"

While this is indeed how you start a roll in an aerobatic plane, doing this for rolling a glider will end up in a dive. It takes a little more to do a proper aileron roll in a glider!

You do a roll change maneuver. In German it is a "Rollwechsel", and the time it takes to do this from -45° bank to 45° bank is an important measure for the agility of a glider. The maneuver is meant to teach you how to quickly change the direction of circling, and for most gliders it limits the minimum size of their vertical tail surface.
The European certification regulations for gliders demand in section CS 22.147:

Using an appropriate combination of controls it must be possible to reverse the direction of a turn with a 45° bank in the opposite direction within b/3 seconds (b is the span in metres) when the turns are made at a speed of 1·4 V$_{S1}$ with wing-flaps in the most positive en-route position, air brakes and, where applicable, landing gear retracted and without significant slip or skid.

Note the speed (v$_{S1}$ is the stall speed): The faster you fly, the easier it is. Since induced drag is highest at low speed, the adverse yaw resulting from the aileron deflection is impossible to overcome if you fly slowly. Glider designers try to make the tail just big enough so CS 22.147 can be fulfilled (the condition "without significant slip or skid" is crucial here!).
If you want to start an aileron roll this way: Please don't! It takes a little more to roll a glider properly.

Here at the University we call it a "Perfect Roll", it is commonly used to assess the turning characteristics of an aircraft.

In Argentina we used to call it "coordinación" (coordination).

Hi Martin, that's exactly how my instructor call it, i'm training in Club de Planeadores La Plata. I haven't fly solo yet, but I fly without the intervention of the instructor, but I couldn't do the coordination excercise right. I'm tryining that skill with Condor Soaring Simulator so I can get it right next saturday. Any tip or sugestion? Thank you!

Please remember that comments should be in English.

It sounds like the process of doing a slow roll, though the pilot usually does the role until they are at the 180 mark, that is inverted.

In addition to being (mistakenly) called a Dutch roll, I hear that maneuver called "rolling on a point" or "rolling on a heading." This is probably the most unambiguous description, as you are rolling the plane while using coordinated inputs to keep the heading constant.

I've heard this maneuver called a "Dutch roll", but that phrase has other meanings too.   It's arguable whether or not this maneuver is even truly possible, or is based on somewhat of an illusion.  If the plane is banked, and you haven't "unloaded" the wing to zero G's, why would the flight path not start to curve to the side, pulling the nose off the point?  To really do this perfectly it looks like you would have to use the rudder in a way that caused significant sideslip, but that was probably not your instructor's intention. (If it was, disregard all but the first sentence of this answer.)

The term Dutch roll is usually reserved for the swaying motion response to a step input in yaw or aileron. What OP describes sounds more like a coordinated turn.


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Could a Dyson fan scale up to be used as a bladeless aircraft engine?
Dyson has been making bladeless fans for a few years now that accelerate a consistent stream of air without exposed blades.

If this concept were applied to aircraft engines, it could potentially reduce the number of bird strikes, engine complexity/part count, danger for ground crews, etc. That said, it has not been done (or has it?). What are the major roadblocks prohibiting engineers from scaling this technology to large aircraft applications?

The theory is to use high pressure flow to generate a high velocity jet, and the jet would induce a lower velocity flow at a higher volume (the x15 number above). Since the purpose for an engine is to generate thrust rather than moving air, the the core of this question becomes "does this high velocity to high volume conversion generate lift" or "is a ring nozzle better than a conventional circular nozzle". I guess answers to both are "No".

The Dyson is just an ejector pump using a electrically driven centrifugal compressor to create the jet, and arranged in an artful way.  It would seem to be a very inefficient way to make thrust, with all the friction losses and such, but it would be interesting to see if the Dyson uses more or less wattage per cfm of air moved than a regular bladed fan (because it's the air being moved that is the thrust).  Dyson's marketing tries to portray his inventions, like the particle separator vacuum design, as groundbreaking, but those things have been used in industrial processes for decades.

Humorously, turbulent airflow is generally perceived as more cooling to the human skin, than smooth or laminar flows too.

@JohnK, Dyson's cyclone separator is impressive in its way.  Industrial cyclone separators are usually pretty touchy about operating conditions, while Dyson made one robust enough to work in a household vacuum cleaner.

In theory, yes, but it'd cost you £300M and only hipsters could fly on it

@RomaH is not an impression. Turbulence increases mixing, and thus heat transport away from the surface.

If you do not know, the Dyson fan is essentially the design of a jet engine's airflow, with the fan repositioned with ducting rather than having the fan jet turbine inline. It could be argued that development of what you have suggested would result in the typical design seen already.

Have you ever used a Dyson fan, and compared it to a decent conventional fan (in same price bracket)? Dyson is a gimmick that puts unusual looks over function. There still is fan intake, it's just placed where you're not looking for it. Safety is even worse, because instead of large, low speed fan, there is small, high speed one that sucks more violently. Roadblocks are irrelevant where given tech offers no benefits nor solves any problems.

@Agent_L Your safety claim is incomplete. The Dyson fan has a small enclosed high-speed fan: higher speed makes things more dangerous but more enclosure makes them safer (in general). You're only considering one of the competing factors.

@DavidRicherby: Other than ceiling fans, I haven't seen a fan that wasn't enclosed anyway. You're right that it's enclosed, but to are the "traditional" fan blades. The only real difference is the speed at which the blades rotate. Higher speed = higher stress = higher break chance (but I do agree that safety isn't the operative word here, but rather durability).

@JohnK I don't know about Dyson's aerodynamics, but the design of their small reluctance motors and brushless motors are pretty revolutionary for consumer products.

@DavidRicherby All desk fans are enclosed in a wire mesh. Conventional fan sucks at low pressure through large area of mesh, through many openings, Dyson sucks at high pressure through small area of mesh, through few openings. I believe that my claim is complete. I compare Dyson fans to desk fans, not to turbofan aircraft engines.

@Agent_L The intake is smaller and in an obscured position, which makes it harder to stick things into it than through the widely spaced mesh of a conventional desk fan.

@DavidRicherby Yes, smaller and in an obscured position, that exactly are the main problems. An intake is safe up in the air, when there aren't any objects, and unsafe when close to the desk surface where small things are stored and dust accumulates. Secondly, the smaller the intake for a given airflow, the harder it sucks air in and is more likely to pick up and ingest foreign objects. If you wonder about people deliberately sticking things into the fan, then indeed Dyson has psychological advantage because the large main loop diverts attention from the actual intake. Not technical one.

Having just had the misfortune to spend 10 days (24 hours/day) in a building where the only aircon was Dyson's so-called "fans", except for a solitary no-brand Chinese-made beast with metal rotating blades (10x less expensive, 10x more effiective), bluntly they are no use for anything whatever, unless you consider them as modern sculptures. My personal impression that Dyson's target market sector is "people with more money than sense who want to support British engineering but know nothing about engineering* was strongly reinforced - they are utterly useless for actually moving air.

Apparently, someone started a company to test this hypothesis. https://jetoptera.com/research-and-development/

No. Not a useful propulsion engine.
The first problem is power. The air stream from Dyson's fans is weaker than what you can get from a conventional fan the same size, and jet engines need a very powerful stream. You'll notice they're quite heavy as well; Dyson AM-06 has a thrust-to-weight ratio of just 0.06, 100 times less than a jet engine.
The second reason is they're very inefficient at high speeds. Dyson's fans use the Venturi effect, which is also used in aircraft evacuation slides, to turn a narrow stream of fast air into a wide stream of slow air. This happens through friction simply pulling slow air alongside the fast stream.
Current airliner engines do the same thing, but better. There, high-energy air spins a turbine, which then spins a fan. The conversion efficiency of this process is 3x-5x better and produces uniform velocity - important when you're close to the speed of sound. The fan approach also results in a smaller, lighter engine.
Jet engines imitating Dyson's fans would need cores and core intakes as least the same size as current ones. The reason Dyson chooses to protect their intake with a grate and Rolls-Royce or GE don't is Dyson fans produce on the order of 50 W of power, jet engines 50+ MW. No one cares about a few watts, they do about a few megawatts, which translate into a couple ocean tankers full of oil over a plane's lifetime.
If you want, you can protect a jet's intake with a grate too - see F-15SE, Su-57 (PAK-FA) with radar blockers, Mig-29's "dirt runway" top intakes. This has nothing to do with using an ejector or not. Dyson's design does remove the slow fan, but at the cost of a lot of weight and efficiency, and it's core damage that is the worst scenario in bird strikes.
The third reason is that Dyson's fans would not scale. To fly at Mach 0.8, the ejector stream would have to be highly supersonic, which is a problem in itself. To use them for hovering, where they are reasonably efficient, you'd face the problem of Venturi effect fading with distance - requiring an entire grate of ejectors, not one simple ring. Helicopter blades do the same thing better, while being far lighter and easier to fold.

+1 for a very well done and comprehensive answer!

I recall reading about someone using this technique to make a flying saucer.  Maneuverability was impressive, but as you note, efficiency was abysmal.

So, in general - a "dyson engine" would weigh much more and be a much more inefficient means of thrust. Both are good reasons NOT to make one. If only "looking totally awesome" were a more meaningful design parameter :D Thanks for the insight @Therac

I found some data on a Dyson fan vs regular fans for CFM per watt, and the Dyson is considerably more efficient than a regular fan, but this is at gentle breeze fan speeds.  The efficiency advantage drops off as you increase the velocity.  This suggests that an "annular ejector thruster"  device that could move enough air to be of use to propel an airplane would be a lot less efficient than driving the air directly.  I would guess that the static thrust of such a device would be not so bad but would drop off a lot as speed increases, pretty much the opposite of a regular jet.

This review shows the Dyson fan peaking at 120 CFM per watt. It would be interesting to see the same figures for a jet engine. Can you convert pounds of thrust to CFM or how does that work?

@JollyJoker You can, multiply volume by velocity by density. Dyson's setting 10 comes out to 1.83 N of thrust for 19 W, or ~96 N/kW. But this thrust is only usable when below 2.78 m/s (flow velocity). N/kW generally scales inversely with velocity, and scaling it to 278 m/s would give 0.1 N/kW. A TSFC=0.6 at 278 m/s, 46% thermal efficiency jet engine is equivalent to 3.1 N/kW.

@JollyJoker Another comparison point could be helicopters. The de Bothezat had to deliver at least 120 N/kW, more than the Dyson's fan. Everything newer is closer to 80 N/kW - but at much higher thrust speeds, even toys start at 6 m/s. So, at static thrust, Dyson's fan is not better than helicopters; it only turns out efficient when compared to cheap desk fans.

Actually, the principle exploited by the Dyson fan is used in some gas turbines, just not quite the way your question expected.
See the section here on “ejector nozzles”.
Here is a picture of the J58 engine. When the tertiary doors are open, air is entrained through them, adding mass flow to the engine. In theory, the extra mass flow gives extra thrust. But, the extra air flow was entrained by the core airflow, so in reality the conservation of energy probably means no extra thrust occurs, because you have a higher mass flow, but some of the velocity from the core flow has been transferred to the tertiary flow, so the average velocity will be lower.
(@John K almost got there when he mentioned “annular ejector thruster”.)

The underlying concept in the Dyson fan may be considered similar to that called: 'Trompe' (See Wikipedia), where a jet of water drags a flow of air. You can have a look in ESPACENET at Patents FR935340: 'Flow enhancer for jets and compressors'; US2946540: 'Jet propelled aircraft'; CA611861, for a 'Flying Disc', all by Henri Coanda, and US2918233: 'Aerodyne', by Alexander Lippisch, all in the same line, thus, I'd consider as best option to accept or reject Dyson fan as an alternative to propellers, ducted fan, or turbofans, conducting some experiments, with the design adapted to propulsion, not cooling or moving air, the Dyson fan data provided in a link in answer by JollyJoker is promising. Not?
The concept in Dyson fan was in some aircraft designs, the Avro Canada Flying Disc never went airborne and was uncontrolable, but the P Moeller disc m200x -

 - flew with multiple fans, and was transposed into a toy named: 'Magic UFO', a variant drone.
See patents: US3065935, J Dubbury; GB383408, C E Johnson; US3022963, JCM Frost, showing the concept in Dyson fan was considered in Aircraft, calculations and experiments have the last word, but if an arrangement moves more air, meaning more mass, at similar speed, meaning more thrust, and this done with less power, men, you have an idea worth testing at airplane scale.
Salut +

please do not post two answers when you want to add informations to another answer of yours


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What are the airworthiness requirements for transport category hybrid airships?
According to section 1.2 of the Airship Design Criteria (FAA P-8110-2), the FAA limits the applicability of airship airworthiness requirements to aircraft "capable of achieving zero static heaviness during normal flight operations."
If that's the case, then under which FAR part are hybrid airships with buoyancy ratios less than 1 certified? Assuming transport category, would the FAA just use Part 25?


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Are QDM and QDR the same as 'inbound' and 'outbound' courses?
I'm about to take my last test to join an airline: the simulator. I've been told they ask to perform some QDR/QDM interceptions, as well as radial ones.
About QDM/QDR:

QDM: Magnetic course from the aircraft towards the station.
QDR: Magnetic course from the station to the aircraft.

I guess we all may agree with that. But:

Could we always take QDM as an inbound course? E.g. the aircraft is at 'radial' 180 and the examiner tells me: "Intercept QDM 320". That would be the same as saying "intercept the inbound course 320", right?
The opposite for QDR. Shall I take it as always an OUTBOUND course?

Probably QDM/QDR is used for NDBs and for practical uses they cannot speak about a radial, but anyways I'd like to know if my points are correct.

QDM is the magnetic bearing to the station.
QDR is the magnetic bearing from the station.

Not to be confused with course, which is the direction in which the aircraft is moving but has nothing to do with the heading of the aircraft. Imagine having a heading of 270 but a course of 090. That means there is so much headwind that you are moving backwards.
Radials are not the same since a radial is always a magnetic bearing from the station. Therefore flying the radial 180 both inbound and outbound will both give you a position south of the station.
This is not the case with QDM and QDR.

Flying a QDM 180 will give you a position north of the station and flying 180 (without taking the wind into account) towards the station.
Flying a QDR 180 will give you a position south of the station and flying 180 (without taking the wind into account) away from the station.

Let's imagine a VOR and an NDB located on top of each other and there is no wind factor:

Flying QDM 180 will put you on radial 000 with a heading of 180.
Flying QDR 180 will put you on radial 180 with a heading of 180.
Flying radial 180 inbound will put you on QDM 000 with a heading of 000.
Flying radial 180 outbound will put you on QDR 180 with a heading of 180.

Long story short, radials are visualised in relation to the beacon. QDM/QDR are magnetic bearings visualised in relation to the plane's nose.


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How to determine aircraft altitude and speed, as a passenger?
Is it possible for a passenger to determine the aircraft's altitude and or speed or location using any mobile apps.
Not all airlines have displays with constant info about this. I had tried using Google maps, but it did not work, maybe because there was no internet/wifi on the aircraft.
Any simple method?

check the GPS on your smartphone

i did have the GPS turned on, but google maps was not indicating any thing. Maybe it needs a data connection.

You can't get location without maps, and you can't get those without a data connection unless you have them loaded on the phone - which in turn means a dedicated app to use them.  GPS will give you speed and altitude though.  Dependant on your phone, you might be able to preload maps.  For example, on my Android phones, I can load maps in Google Maps then make them available for offline use but that will depend on phone, OS and version.  I've never tried this for the areas of maps needed to track a flight though.  YMMV.

CloudAhoy is a great app for pilots that you might find interesting. It is free and you can see a ton of information about your flight. http://www.cloudahoy.com/

or get an app that can show you the location/speed/altitude stats without the map data.

@simon you can get location with a generic gps app and no data connection if you can manage to see enough satellites through the window. You only need data if you want to get a map of you location.

@Firee, yes, Google maps need data connection to download the map tiles. You need either a pure GPS app that will simply show readings from the GPS (e.g. GPS Essentials) or a navigation that uses offline data including several that use the free OpenStreetMap data.

I just used a standalone car navigation system. It displays the location on the map, but also in the status page: longitude latitude, speed, heading, and height. Very handy to put a name on lights or lakes below. First time I saw the GPS displaying 799 km/h :-)

@casey Er, that's exactly what I said ;)  Ah, I see what you mean.  I meant "location" as show me on a map. Of course you get lat long without doing anything.

flightradar24 has apps version which allows you to access aircraft altitude and speed. But it cost some money (but webpage version doesnt) and wifi connection is needed.

I always track my flights, so here is my experience:
App using offline maps
As it has been said, you will need an app which can download maps prior to the flight. I never had a flight with onboard wifi, but I guess, there's usually a data volume constraint, which could make it expensive.
I use Mapfactor Navigator (Android), which is an offline navigation system for cars etc.
The maps can be obtained from within the app, the user has the choice between free maps from the OpenStreetMaps project and TomTom maps, which you have to pay.
The maps can be downloaded per country, and big maps are subdivided into smaller pieces. For example, the very detailed map for Germany is divided into four parts, together about 0.8GB of data. The entire USA can be downloaded per state, all together 1.7GB.
It's possible to configure several fields at the bottom of the screen to display speed, altitude, GPS coordinates, distance to destination and so on.
I mainly use this app to explore a country (e.g. by car) on vacation, but it's also fun to use it on board to see what's that location below us.
You may switch off speed camera warning. I once got a warning over Switzerland. The 840km/h were well above the max speed limit of 120km/h, and the fine should have been in the order of seizing all property of the airline. For some reason, the pilot didn't care...
OK, lots of advantages, but there's also a drawback: The app can not just record your flight, though a menu for tracking exists. Nor can it display your currently covered track. It can only record a track when it's navigating you, but you don't want to hear "turn around! turn around!" all the time...
Tracking app
Again, there should be thousands of apps out there. I'm using Open GPS Tracker, which can also display your position / track on an online map. So, you just see a point moving on a gray screen while flying.
However, this app allows to record your track, with up to one point per second, and at the end, you can export it as GPX file, which is the standard. It can be opened by many map tools, also by googles MyMaps or what it's called. The file contains coordinates, precision, time, altitude, speed, number of satellites and course.
One hint: This app makes a sanity check between speed reported by the GPS module and speed calculated from recorded points. If this fails, it doesn't display/record any speed. This often happens onboard, so switch this check off.
Here is a flight from Düsseldorf (DUS) to Las Palmas, Gran Canaria (LPA), including altitude profile, drawn with Marble (Linux KDE program):

Precision
An airliner is usually made of aluminum, which makes it a huge faraday's cage. The reception of any radio waves is very limited (through the windows), and so is GPS.
And as GPS needs a line of sight to several satellites spread as much as possible over the entire sky, the only chance is to have a window seat. There, I usually get a very good GPS signal, while it's already very bad to unusable on the next seat, and don't even think about it on the aisle seat.
Having a window seat, I usually get a good signal. While taxiing, the precision is quite good, as on this flight from Frankfurt (FRA):

Of course, the strength and accuracy of the GPS signal relies on the quality of the GPS module of your phone. My last phone wasn't so good, I often had jumps of several 100m sidewards, or from 7000m to 10000m altitude. This is just because accuracy comes with the number of satellites, which usually increases with altitude. And: The first aim of GPS is to locate your position, the altitude usually is less important. With my current phone, I don't see such problems. No sudden steps aside, and altitude and speed were quite the same as displayed on the entertainment system.
Well, not entirely. I noticed my phone showing an altitude about 50m too high when landing on my last flight. The 20m shown by the entertainment system were plausible, as the airport was nearly on sea level / in the sea. But the too high altitude was still there, even at the beach. Keep in mind the earth isn't a sphere, it's an ellipsoid, though even that's not correct, it's more like a potato. So, sea level isn't the same everywhere. High quality/price GPS receivers know this and contain data to compensate it, but cheap ones just use the ellipsoid model. (However, 50m is nothing during flight, and what do you expect from a phone inside the cabin...)
More hints
For a working GPS, the receiver needs a list of all GPS satellites and their precise orbits ("almanach"), current time and its rough position. If it does not have its rough position, it takes much more time to get a fix (first GPS position). And if it doesn't have a recent almanach, it has to listen to the satellites for a while, as they slowly broadcast it.
As reception of radio signals is quite difficult inside the cabin, it may be very hard to get a fix, and even impossible to get the almanach.
So, it's good practice to prepare your phone before the flight, as reception will be better, and assisted-GPS allows to retrieve the almanach and current position via the mobile network.
Finally: The battery.
Running my two apps on a 4.5h flight takes about 80% battery, and I have a laaarge battery. Of course, this also depends on display-on time. But the endurance could by tested before the flight, and if it's too short, a power bank my help.

My experience is that the display takes more power than the GPS, so it really depends on how much you are looking at the data rather than just recording. And of course don't forget to put the phone in airplane mode, the radio module is the biggest power hog.

Does GPS really distinguish between altitude und position? I always thought GPS just gives you a timed 3D position and the rest is done by your local software by mapping the GPS position onto a model earth, where "altitude" is just a fancy word of saying "z component in relation to EGM96".

@hiergiltdiestfu: I'm not sure I understood you correctly. GPS determines the distance to the center of the earth, and using the known local sea level, it calculates the altitude (above sea level). The question is what data / model is used to define local sea level. Is it just an ellipsoid (like WGS84) or a more sophisticated one like EGM96? This may also just depend on the GPS device itself.

Thank you for your reply, sorry for not being clear. I was referring to this part of your answer: "The first aim of GPS is to locate your position, the altitude usually is less important." I'm just trying to point out // ask whether it's actually valid to distinguish between altitude and position, when "altitude" is actually only a component of the 3D fix GPS does and only becomes "altitude" when translated onto some model of the earth. Or is it?

@hiergiltdiestfu: You can increase accuracy of one parameter if you allow a lower accuracy of another. If you use satellites just above the horizon, you can determine your (2D) position very precisely, but not the altitude. If you also use satellites above you, you'll also get a precise altitude. So, if a GPS device can handle just a few satellites, it would mainly use those above horizon, because people are more interested in position than altitude.

There's a great app for that info called GPS Status & Toolbox It gives you speed, altitude, longitude and latitude from your GPS. It will also give you pitch, roll, heading and acceleration data. None of this needs a data connection, just a view of GPS satellites. Never tried to get GPS from inside a plane, though. Not sure how well that works.
If you want to see your location on a map, if the plane doesn't have wifi, you'll have to download an offline map app. There's several of them available. The Google maps offline feature wouldn't work, really, because it only lets you download a certain size of tile. It took four such tiles to include the entire metro area of my home city (KC). It's pretty data heavy. Rural areas with less data can get a somewhat bigger tile, but if you're flying very far it would take forever, a lot of effort and memory space to download enough tiles to cover the whole trip offline.

Thanks for the GPS Stauts & Toolbox reminder. Had it ages ago on my first 'droid, and it's quite handy, especially for calibrating the phone's sensors. I'd forgotten all about it!

This occurs pretty frequently in the GA community as apps like ForeFlight are quickly becoming the preferred method of navigation in small and even larger planes. There are a few problems that need to be overcome and ill outline them here.
Offline: Since you wont have access to the internet in the air you will need to cache the maps offline. There are lots of apps out there that do this in a variety of ways for a variety of reasons. Some are for hiking, others driving, and many for flying specifically. ForeFlight, FltPlanGo, and Garmin Pilot are just some of the ones that allow offline flight maps to be stored. Some of these apps require expensive subscriptions but they ensure that your data is always legal and up to date. If you are not a pilot FltPlanGo (free) is a cool one to check out. Flight based apps will also allow you to see cool info related to your flight like weather, approach plates and overlay over airspace maps instead of traditional road maps. ForeFlight even knows when you land and will pull up the airport diagram for you!
GPS: One of the major problems you will have is the GPS chip thats internal to the device. On one hand you have the fact that the airplane is a giant faraday cage as mentioned in other answers. This prevents you from getting a decent signal but even if you can get a good signal you will be plagued by the not so great GPS chip in most phones. This Article goes over it nicely

When using the iPad with only the internal GPS in your lap in the cockpit, we’ve found that the GPS tended to lose signal reception intermittently. Sometimes the iPad will not lock onto GPS again until you reboot your iPad.

but in general I have found the iPhone and iPad GPS to be less than optimal when I'm flying even small GA planes at far slower speeds. One of the best solutions to this is to get an external GPS receiver like this one they are small and provide great reception at all altitudes. Granted its a bit pricy just to know where you are on a commercial flight but it will do what you need it to.

(source)
INS: Your other option (and most airliners actually use this to navigate in some capacity) is to rig up a simple inertial navigation system with your phone. The issue this will induce is that you wont really be able to use your phone unless you don't move it. Using this method you can use the internal accelerometers to measure acceleration and calculate velocity in all 3 dimensions and inevitably deduce position from all that. There is at least one android app I can find that does it or you can (if you so chose) write your own. Along with not being able to move your phone INS systems also suffer from compounding errors which make them increasingly inaccurate as time goes on.
Dead Reckoning: This is one of ways pilots are taught to navigate and to some extent you could do it from your seat with a stopwatch, compass and map. If you know what plane you are on and you know its approximate speed (based on published numbers) you could compute your path. Practically speaking you will also need to know the winds aloft since your compass will read out the way the nose of the plane is pointing not the way its moving over the ground. Aloft you can easily have winds in excess of 50Kts or even 100Kts, if thats a direct crosswind your heading and ground track will be substantially different. As long as you know what the winds aloft are (For the US they can be found here) you can fill in the blanks.


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What happens if a seat belt is found broken, before flight take off?
What happens in a commercial airliner, when a passenger finds the seat belt to be broken. Does the crew de-plane the passenger or do they provide him something like a child/infant seat belt?
I'm also curious as to FAA's regulation on this area, if any.

The infant seat belt is attached to the actual seat belt, so not an option (besides being way too small). Regulatory the seat belt needs to be there, so I assume if there is no other free seat the passanger will be either removed from the plane or one of the pilots takes them on their lap.

What about the seat belts used by crew during safety demonstration?

@Firee If you look closely, you will see that all of the equipment used in the safety demo is marked "for demonstration only" or similar since it is not maintained.

F/A jumpseats might be usable in some jurisdictions I think.

The seatbelt is either replaced with a spare or the seat is "deferred" (put out of service) until it can be repaired.

This happened to me once. The crew relocated me to another seat. You cannot fly without the basic security gear.

And a corollary question: What happens if a seatbelt on a full aircraft becomes inoperable during flight?

Sounds like the best thing to do (if in a hurry) would be to report it after takeoff. Do seatbelts on aircraft save lives anyway?

Options would be to either move the passenger to another seat & placard that seat as unusable, or go back to the gate and have maintenance fix what's broken. In the former case, the seat would most likely be fixed by a mechanic at the next station.
The seatbelts used in the F/A demos are seatbelt extensions, which still need the installed seatbelt itself to be intact. They are fully functional as such, though, unlike the demo life vests & O2 masks, which indeed are "demo only" and of no use at all in a real emergency.
For a mechanic, the seatbelt is quick & easy to replace, although without needle-nose pliers (and a spare male or female belt), rather difficult for the crew -- who wouldn't be allowed to anyway.
The FAA requires that every passenger has a seatbelt and has been told how & when to use it, and that the crew has verified that they have it on before takeoff.

Also, the crew would have a hard time doing the required maintenance paperwork, including returning it to service...


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Are there different configurations of cabin crew seating arrangement for one aircraft model?
In many airlines, cabin crew are shown to be sitting in different areas. Is it common for airlines to offer different seating configurations for a particular airline model? Also, are there any advantages of the same?
For example, in A320, you will find cabin crew seating in the aisle at the rear of the aircraft in some aircraft, near the toilet.

Crew seating is partly concerned with the regulations requiring a crew member next to certain exits. Airlines choose different combinations of seating based on class and galley/lavatory arrangements and this changes the number of required exits and crew required to sit next to the exits. Some long hall airlines require berths for crew rest and sleep - I imagine some airlines may have employee union requirements.
All this requires ongoing customized cabin arrangements. Airlines often change the seating during mandatory maintenance (a, b, and c checks) to accommodate different routes, cultural expectations and changing economy.
Interestingly, the cockpit itself may vary. According to FAA regulations large aircraft must have at least one observer seat (usually called a "jump seat"). But, I have seen cockpit options that vary the number and type of "jump seat" in the triple 7.
For example, one cockpit of a Boeing 777 offered two padded conventional seats or three fold down "bench" type seats. I guess one company wanted the FAA observer to fall asleep and another company wanted to discourage "observers".


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Why are digital autopilots better than analog?
I've heard many people say that a digital autopilot is better than its analog equivalent. How do the two differ in design and the way they control the plane?

A lot of people think that anything newer is automatically better...

Digital is NEVER better. Digital is just different. Because digital systems are usually newer than analog ones, they just often tends to be better.
Look at audio files: MP3 has yet to match the quality of a vinyl, but yet it has many advantages over it.
And many computers rely on analog devices as source of informations (sensors, ...)

@antzi, I'd have to say that is completely wrong. Digital is more flexible than an analog system because you can do far more with code than you can with discrete elements of a circuit. Your audio example describes the difference between a continuous signal and a discretely sampled signal. That has nothing to do with the quality of digital vs analog.

@antzi: Good luck making an analog GPS unit.

@chris12892 nonsense. Digital systems have many problems analog problems avoid, especially in the discrete nature they are by force required to adopt. Analog systems are usually continuous, meaning changes over time are more correctly defined.

@Antzi "Digital is NEVER better." That is a very incorrect statement. Digital signals are generally much more error/interference-resistant than analog ones and, for that reason alone, are very often better than analog signals. Digital signals are very often designed such that there will be no data/precision loss at all until such time as the entire signal is unrecoverable, which usually results in much better quality reception than an equivalent analog signal, even when the analog signal is using more bandwidth. See NTSC vs. ATSC or FM vs. digital radio, for example.

Early autopilots would only control one degree of freedom. Starting with the yaw damper of the Me-262, they were a simple feedback loop involving a gyro, a single control surface and some damping circuitry. Example: A roll gyro was coupled to the ailerons and kept the wings level. Commanding a bank angle would simply shift the zero point of the gyro to a different angle, and some damping made sure that the movement was slow and did not overshoot.
This, however, would neglect any cross coupling. Banking flight needs more thrust to keep altitude, but the roll autopilot would not "know" of engines or throttles, just ailerons. The same was true for analogue pitch and altitude hold systems which had a simple feedback loop with the throttle and the elevator, but no other controls.
Modern digital autopilots, starting with the one in the X-31, control all axes in parallel and use a multitude of sensors and feedback loops to control the whole aircraft in one go. This is much easier to achieve with digital control, and gives smoother responses since cross-coupling effects can be anticipated and corrected before they manifest themselves.

@jjack: This is new to me, and considering that it flew mostly at low level, had a decent fuselage length and only a top speed of 400 km/h, I wonder why it should have needed one.


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How often is a passenger jet aircraft painted?
I just read an article (published in 2012) telling the special livery for British airways' A320 to celebrate the Olympics game is made to last one year. I assume this mean those aircraft are painted once per year because painting is not free and thus it is reasonnable to change the livery only when painting is already schedule. But because this is a special livery, it may last less than other painting. As this is only assumptions, my question is the following:
How often is an Airbus or a Boeing painted (even without changing the livery)? Does it significally change from on airline to another? Does a special livery change this frequency?

Its also worth noting that paint is heavy, and hugely affects aerodynamics.

Seems like the paint weighs about 1100 for a 747.  Depending on if they completely strip the old paint or not, the weight could add up pretty quickly.

@KeeganMcCarthy I suppose the old painting is completely striped when an aircraft is repainted (less weight, begin the painting with a clean surface) and the painting is made wuth specific technics by specialized workers to limit weight and aerodynamic secondary effects.

As always, it depends. From what I've seen and read, it's usually redone every 5-10 years. This can vary on various factors from paintshop availability, to new color scheme rollout, to deterioration of the previous one due to bad painting, to quality of the paint used. Sometimes, special liveries are just an aviation specific sticker that's put onto an aircraft, other times it's fully painted onto the aircraft, just depending on the airline, the planned length of time for it to be special, and any other considerations.

Operator changes -> aircraft usually repainted. Mergers (UAL/COA, DAL/NWA, AFR/KLM, AAL/USA...) Events (World Cup, Dubai Shopping Festival, F1 Grand Prix, Santa...) Adverts and/or agressive com (RYR, EZY...) Airlines like American Airlines or Alitalia hasn't changed them for decades, while airlines like Air France or Qantas only made slight changes. Angola Airlines declined the Boeing proposal so two airframes (B772ER and B737) to be delivered has to be painted back to the original scheme. In the end, it highly depends on the airline as said.

Do you have any references to add in your answer?

Aircraft on the ground don't make money, so airlines only ever re-paint their aircraft when needed.
It is a regulatory requirement that an aircraft undergoes a thorough inspection - a "D Check", approximately every 6 years. As part of this D-check, the paint is stripped off the aircraft to allow the hull to be thoroughly inspected. As such, most airlines will wait until the aircraft is due for a D check before repainting.
Apart from that, they will only repaint when absolutely necessary, i.e if the paint is deteriorating, or if they change livery and decide they want to remove the old livery quicker than waiting for aircraft to require a D check.